The B cell antigen receptor (BCR) enhances the presentation of antigen to T cells over 1000-fold by, in part, delivering antigen to specific compartment(s) within the cell where it is processed for loading onto MHC class II. A newly identified intracellular structure, antigen receptor induced subcellular complex (ARISC), is seen in a mouse B cell line following antigen receptor (BCR) engagement. The principal investigator and his colleagues propose that: 1) The ARISC is necessary for the BCR facilitated presentation of some antigens and 2) The ARISC is a consequence of one or more signals that emanate from the BCR and that these same pathways are utilized by the Nef protein. These hypotheses will be tested in the aims 1) Characterize the ARISC and establish its significance in vivo, and 2) Elucidate the signaling mechanisms involved in ARISC formation.